1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a thin film transistor array substrate and a method for manufacturing the same, in which it is possible to prevent mobile ions contained in a substrate from penetrating into a semiconductor layer by the gettering effect or neutralization in case the substrate is formed of Low End Glass.
2. Discussion of the Related Art
With the development of an information society, demands for various display devices increase. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), and vacuum fluorescent displays (VFD). Some types of the flat display devices are already in use in displays of various types of equipment.
Among the various flat display devices, the liquid crystal display (LCD) device has been most widely used due to the advantageous characteristics of thinness, lightness in weight, and low power consumption. In this way, the LCD device substitutes for the Cathode Ray Tube (CRT). In addition to the mobile type LCD devices such as displays for a notebook computers, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
The general LCD device includes an LCD panel for displaying an image and a driver for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates bonded together with a gap therebetween, and a liquid crystal layer injected between the first and second glass substrates.
The first glass substrate (TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes and a plurality of thin film transistors. The plurality of gate lines are formed on the first glass substrate at fixed intervals in one direction, and the plurality of data lines are formed at fixed intervals substantially perpendicular to the plurality of gate lines. Then, a plurality of pixel electrodes in a matrix arrangement are formed in pixel regions defined by the plurality of gate and data lines crossing each other. The plurality of thin film transistors are switched according to signals on the gate lines for transmitting signals on the data lines to the respective pixel electrodes. The second glass substrate (color filter substrate) includes a black matrix layer preventing light from leaking into regions except the pixel regions of the first substrate, the R/G/B (red/green/blue) color filter layer displaying colors, and a common electrode displaying a picture image.
Next, a gap is maintained between the first and second glass substrates by spacers, and the first and second substrates are bonded to each other by a seal pattern having a liquid crystal injection inlet. At this time, the liquid crystal layer is formed using a liquid crystal injection method, in which the liquid crystal injection inlet is dipped into a container having liquid crystal while maintaining a vacuum state in the gap between the first and second glass substrates. That is, the liquid crystal is injected between the first and second substrates by an osmotic action. Then, the liquid crystal injection inlet is sealed with a sealant.
Meanwhile, the substrate for the LCD device is formed of a transparent glass substrate. The transparent glass substrate should be non-alkaline, heat-resistant and chemical-resistant. More particularly, the alkaline elements of the transparent glass substrate cause the thin film transistor to deteriorate. Therefore, it is necessary to completely remove the alkaline elements such as the sodium or potassium group materials in the transparent glass substrate. Also, the substrate is repetitively heated and cooled during a plurality of semiconductor process steps on the substrate. In this respect, it is important to obtain the heat-resistance characteristics in the substrate to stabilize the size of the substrate in case the glass substrate is heated or cooled. Also, a process for etching a metal or an oxide is required to form a driving device on the substrate for a display operation. The substrate has to obtain stabilization for an etchant of the metal or oxide.
In manufacturing devices using glass substrate, various kinds of glass substrates are used according to a manufacturing method as follows.
For example, a simple matrix type LCD device is manufactured at a low temperature of 300° C. or less, whereby lower and upper glass substrates of the said LCD device are formed of Soda Lime Glass of the inexpensive Low End Glass group. However, in case of an active matrix type LCD panel having a polysilicon type thin film transistor, a high temperature process of 300° C. or more is performed to lower and upper glass substrates. Thus, the lower and upper glass substrates are formed of Alumino-Silicate Glass having great resistance to heat damage or temperature changes and etching-resistance characteristics. The Alumino-Silicate Glass is three times as expensive as the Soda Lime Glass. That is, as the size of the LCD device increases, the cost of manufacturing material such as substrate is a burden.
The following table shows the remaining amount of sodium ions according to the kind of the glass substrate.
TABLE 1Kind of Glass SubstrateStructureAlumino-Silicate GlassSoda Lime Glassn+/SiNx/GLSa-Si:H1.3 × 10−31.9 × 10−2(250° C.)SiNx2.1 × 10−32.6 × 10−2n+/SiNx/BCB/a-Si:H5.5 × 10−41.8 × 10−3GLS(250° C.)SiNx1.4 × 10−33.0 × 10−3BCB(Na/C)2.2 × 10−36.7 × 10−2ITO/GLSITO(Na/In)8.5 × 10−35.3 × 10−2GLSGlass3.2 × 10−13.6n+/SiNx/GLSa-Si:H7.3 × 10−41.3 × 10−2(320° C.)SiNx——
As shown in Table 1, in the case of the Low End Glass such as the Soda Lime Glass, respective glass substrates contain a large amount of sodium as compared to those of the Alumino-Silicate Glass. Even though the temperature of heat treatment is changed in case of the Soda Lime Glass, the amount of sodium ions is scarcely decreased. In fact, the amount of mobile ions remaining on the semiconductor layer (a-Si:H, before hardening to a polysilicon) is the most serious problem because the semiconductor layer obtains TFT device characteristics.
Accordingly, the substrate may be formed of the Soda Lime Glass in a simple matrix type LCD device since it is possible to contain mobile ions in a buffer layer of silicon nitride layer SiNx formed on the glass substrate. However, in case of the active matrix type LCD panel having an amorphous silicon type thin film transistor requiring a high temperature process, it is impossible to use the buffer layer as a barrier layer because the great amount of mobile ions, so the Soda Lime Glass is not used as the substrate in the active type LCD panel.
Hereinafter, a method for manufacturing a thin film transistor array substrate of an LCD device according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view illustrating a semiconductor layer on Low End Glass in an LCD device according to the related art. FIG. 2 is a cross-sectional view illustrating mobile ions of the Low End Glass penetrating to a semiconductor layer.
As shown in FIG. 1, in the LCD device according to the related art, a buffer layer 12 is formed on a glass substrate 11. The buffer layer 12 serves as a barrier layer. Then, a semiconductor layer 13 is formed in an island-shape on the buffer layer 12. The buffer layer 12 prevents mobile ions of the glass substrate 11 from penetrating into the semiconductor layer 13.
Although not shown, a gate insulating layer is deposited on the buffer layer 12 including the semiconductor layer 13, and a gate electrode is formed on a predetermined region thereof. Subsequently, an insulating interlayer is formed on an entire surface of the glass substrate 11, and source/drain electrodes are formed and connected to a impurity region of the semiconductor layer 13, thereby forming a thin film transistor. After that, a pixel electrode is formed and connected to the drain electrode, whereby the thin film transistor array substrate is completed.
However, as shown in FIG. 2, in case sodium ions Na+ flow into the glass substrate 11, the buffer layer 12 does not serve as the barrier layer preventing mobile ions from penetrating into the semiconductor layer 13. Especially with Low End Glass containing a large amount of mobile ions, the buffer layer 12 does not serve as the barrier layer for preventing penetration of mobile ions to the semiconductor layer 13. Thus, the gate threshold voltage is increased due to the mobile ions penetrating into the semiconductor layer 13.
The method for manufacturing the thin film transistor array substrate according to the related art has the following disadvantages. Recently, the thin film transistors are manufactured at a low temperature of 200° C. or less, so that it relaxes the requirements on heat damage or temperature changes. Accordingly, if the substrate satisfies requirements except the heat-resistance requirement, the Soda Lime Glass may be used as the substrate for the LCD device. It is preferable to use the Soda Lime Glass as the substrate because the Soda Lime Glass is inexpensive. Soda Lime Glass costs one-third as much as Alumino-Silicate Glass. So if it is possible to prevent the semiconductor layer from being contaminated by the mobile ions and impurities of the glass when using the substrate of the Soda Lime Glass, the manufacturing cost will be greatly reduced for the LCD device. However, the buffer layer, formed between the semiconductor layer and the substrate, does not serve as a barrier layer, whereby the mobile ions of the substrate may penetrate into the semiconductor layer. Thus, the gate threshold voltage is increased when forming the thin film transistor.